Recovering Channel Amplitude Reciprocity with Data-Aided Gain Imbalance Estimation in an SFLC-OFDM System

This paper proposes a practical scheme for estimating and compensating channel amplitude asymmetry in a SpaceFrequency Line Code (SFLC) - Orthogonal Frequency Division Multiplexing (OFDM) system. SFLC assumes channel reciprocity using Time Division Duplexing (TDD) for estimating downlink (DL) channel state information via uplink (UL). However, in practice, channel amplitudes between UL and DL are asymmetric, causing unexpected phase rotation in decoded symbols. To address the issue, we propose a data-aided gain calibration technique to maintain channel amplitude symmetry and improve the performance of SFLC decoding. We design an estimator of the phase rotation based on a mathematical model. We then propose a calibration scheme that compensates for channel amplitude asymmetry by adjusting the receive gain of an antenna. We also provide experimental results with implementation on a software modem. A testbed using USRPs is established, and experiments with real-time signal processing are conducted to verify the functionality. The results show that the phase rotation is properly estimated and effectively nullified via the proposed scheme.